The present invention relates to methods and apparatuses for testing the response of a structure to an explosive event, more particularly for testing the response of a submerged hull structure such as a submarine to an underwater explosive event.
During a typical underwater explosion (UNDEX) test, the target is a hull model which is initially struck by a shock wave. Typically, the shock wave results from conversion of about half the chemical potential (explosive charge) energy into kinetic energy in the water surrounding the charge. The explosion products form a bubble which expands to maximum size in a span of xcx9c100 times the time constant of the steep fronted, exponentially decaying, free field incident shock wave. The shock wave response of the target to this later, more slowly applied pressure load is characterized by lower frequency and longer wavelength motion. This is in comparison with the shock wave response of the target to the earlier, more rapidly applied pressure load which is characterized by higher frequency and shorter wavelength motion.
Submersible hulls are tested, particularly with respect to internal/external equipment survival, in underwater explosion environments. This testing includes UNDEX model testing, often at reduced scale, but in some cases at full scale. Various test vehicles (xe2x80x9ctargetsxe2x80x9d) have been designed, fabricated and tested over the past half century. Most of these have been short (length/diameter ratio of xe2x88x921), therefore responding primarily in early shock deformational modes involving higher frequencies and shorter wavelengths. Longer models (length/diameter ratios of xe2x88x929) have been employed when special circumstances have demanded additional kinds of response, such as the bending (xe2x80x9cwhippingxe2x80x9d) motion associated with later shock deformational modes involving lower frequencies and longer wavelengths. Such vehicles, even at reduced scale, but large enough to allow inclusion of essential details, can become heavy (e.g., about 64 long tons dry with about 80 long tons displacement), and expensive (e.g., about two million dollars).
For a particular project, the inventor and his colleagues considered a mechanically excited (e.g., via impact) xe2x80x9cdry landxe2x80x9d approach. However, such approach was dismissed as untenable in view of the huge mass required to simulate the dynamic participation of the adjacent ballasting structure and fluid in addition to that of the hull test section itself and the equipment within. Other factors also pointed to the preferability of a xe2x80x9csubmergedxe2x80x9d approach to testing. According to a xe2x80x9cdry landxe2x80x9d approach, the simulated fluid xe2x80x9caddedxe2x80x9d mass would have to be absolutely devoid of shear stiffness, a difficult proposition. Furthermore, it would be difficult to simulate UNDEX loading xe2x80x9cin the dry.xe2x80x9d
In view of the foregoing, it is an object of the present invention to provide method and apparatus for simulating submarine hull target response to UNDEX (underwater explosion) excitation.
Another object of the present invention is to provide method and apparatus for measuring both high and low frequency response components to UNDEX load in submersible hulls and equipment.
A further object of the present invention is to provide method and apparatus, characterized by reusability, for deducing velocities and stresses in submarine hulls and in internal equipment for purposes of assessing the survivability of novel or extant hull, equipment or equipment-support designs.
Yet another object of the present invention is to provide method and apparatus, characterized by cost-effectiveness, for determining hull and equipment UNDEX response and survivability.
In accordance with typical embodiments of the present invention, a vehicle comprises three hollow, axially aligned, axially symmetrical sections, viz., a hull section and two bellows sections. The hull section has two hull section ends. Each bellows section generally describes a peripherally (e.g., approximately perimetrically or approximately circumferentially) pleated shape and is attached at a hull section end. The vehicle is adaptable to use in association with explosion means for testing response to underwater explosion. Each bellows section attributes the vehicle with axial flexibility responsive to the underwater explosion.
The terms xe2x80x9cbellowsxe2x80x9d and xe2x80x9cconcertina,xe2x80x9d as used herein, each synonymously refer to any apparatus generally characterized by a geometric axis and a plurality of generally parallel and generally peripheral (e.g., perimetric or circumferential) folds, bends or pleats which attribute the apparatus with a degree of flexibility in the generally axial direction. A typical bellows or concertina apparatus in accordance with the present invention is analogous to a bellows or concertina apparatus which is included in, part of or associated with a type of musical instrument commonly known as an xe2x80x9caccordion.xe2x80x9d
In accordance with the present invention, a vessel is provided which may be used as a test model for evaluating the response of a full-scale version thereof to an underwater explosive event. In particular, a submarine test vehicle is provided by the present invention to determine both early (high frequency) and late (low frequency) UNDEX hull and equipment response. Of particular note is the present invention""s capability of determining late (low frequency) UNDEX hull and equipment response. Associated with late (low frequency) UNDEX hull response is a hull xe2x80x9cwhippingxe2x80x9d motion. In the past, when xe2x80x9cwhippingxe2x80x9d motion required study, long models (e.g., length/diameter ratios of xcx9c9) were employed. As previously pointed out herein, such vehicles, albeit at reduced scale but nevertheless large enough to enclose essential details, tend to be massive and costly. The present invention""s test vehicle can be excited, without damage, up to design severities, in xe2x80x9caccordionxe2x80x9d modes previously inattainable in any vehicle having a length/diameter ratio as low as 3. Hence, the present invention""s test submersible is typically characterized by a relatively low length/diameter ratio, and yet affords test information comparable in value to that afforded by a conventional test submersible characterized by a much higher length/diameter ratio (e.g., xcx9c9) as well as realistic flexural/longitudinal modes. Accordingly, the present invention is a xe2x80x9cshortxe2x80x9d UNDEX model whose submerged vibration characteristics simulate those of a xe2x80x9clongxe2x80x9d prototype.
The inventive submarine model vehicle subjected to inventive testing has been dubbed by the inventor the xe2x80x9cPoisson Blancxe2x80x9d (PB) in contradistinction to a like diameter generic submarine pressure hull model prototype which is three times longer, named the xe2x80x9cWhitefish.xe2x80x9d The inventive testing demonstrated that the inventive Poisson Blanc""s response to underwater explosion loading simulates or mimics that of the Whitefish. The inventive PB thus represents a dynamic surrogate of the longer prototype. The inventive PB""s middle or central part is a generic ring-stiffened (e.g., cylindrical) pressure hull test section, of arbitrary design, which houses equipment. At each end of the middle test section is a perforated (bolt ring) flange. Bolted to each flange is a xe2x80x9cconcertinaxe2x80x9d or xe2x80x9cbellowsxe2x80x9d apparatus which is just over a quarter of the test section in length. Each bellows apparatus has two manhole-equipped (hatch-equipped) end plates (bulkheads). Further, each bellows apparatus has one or more valves (located at the outboard bulkheads, only) for intake and scavenging (expulsion) of liquid (e.g., water) or gas (e.g., air). Accordingly, the bellows (concertina) apparati pair provides for: (i) submergence (diving) ballast for the inventive model vehicle; and, (ii) the combination of low stiffness and large inertia of the inventive model vehicle, thereby together enabling low frequency bending (i.e., axial and bending, or according to this invention xe2x80x9cbeam/accordionxe2x80x9d deformation) to take place. The UNDEX loading external to the inventive PB vehicle is measured by pressure gauges, while response measurements of the vehicle and the equipment under investigation are obtained by means of strain gauges, relative displacement gauges, force gauges, velocity meters and/or accelerometers.
In accordance with the present invention, each xe2x80x9cconcertinaxe2x80x9d behaves in a manner analogous to that which the name implies. When a musician plays a musical instrument known as a xe2x80x9cconcertina,xe2x80x9d the musician""s hands translate 180 degrees out-of-phase, alternately compressing and expanding the concertina""s bellows. Thus, each concertina bellows manifests both axial compressive action and axial tensile action. While this is occurring, the musician""s hands simultaneously rotate. The combined effect of all of this activity is both axial (compressive and tensile) motion and bending (lateral) motion of the bellows. Bending motion of the concertina bellows would occur simply by virtue of the interaction between compression and expansion of adjacent xe2x80x9cpleatsxe2x80x9dxe2x80x94that is, even in the absence of rotation of the musician""s hands. Pure bending, in isolation, will result from the simultaneous conditions of (i) the compression of a first set of pleats and (ii) the expansion of a second set of pleats which, in function or effect, are xe2x80x9cdiametrically oppositexe2x80x9d the first set. Bellows movement, therefore, basically consists of a combination of bending and axial components.
The principles elaborated upon in the preceding paragraph are applicable to the present invention""s Poisson Blanc. In inventive practice, a distinction is drawn between: (a) the excitation of the middle test section by one or both concertina sections; and, (b) the response of the middle test section to such excitation by one or both concertina sections. While the middle test section""s excitation is both axial and flexural in nature, the middle test section""s response is primarily flexural. This situation essentially results from the very high axial stiffness of the middle test section. In fact, the middle test section does vibrate axially, but at much higher frequencies. Since the axial vibrations of the middle test section will generally be at such high frequencies, they will generally not be of great significance in the context of inventive practice of UNDEX experimentation. Generally, although the inventive practitioner will obtain both axial and flexural responses, the flexural frequencies will be important, whereas the axial frequencies (which will usually be of about an order of magnitude higher than flexural frequencies) will not be important. Of main concern in typical embodiments of the present invention is the ability to obtain correct flexural response from the coupled axial/flexural concertina motion.
As an aside, the axial displacement of a row of individual leaves is a fair illustration of inextensional bending. The term xe2x80x9cinextensional bendingxe2x80x9d refers to a class of plate and shell response problems in which the potential energy is dominated by flexural strains as opposed to extensional strains. Inextensional bending is of little import in the present invention, however, as behavior xe2x80x9cin the largexe2x80x9d is of greatest interest in inventive practice.
The present invention thus provides a unique UNDEX test vehicle. The present invention""s vehicle is capable of being excited, without damage, up to severities at design values, in xe2x80x9caccordionxe2x80x9d modes heretofore unattainable in any UNDEX test vehicle having a length/diameter ratio as low as three. The low ratio value of approximate magnitude three for the present invention""s submersible device contrasts markedly with the usual ratio values of approximate magnitude nine for submersible devices possessing significant flexural and longitudinal vibration modes. Thus, the present invention is a xe2x80x9cshortxe2x80x9d UNDEX model having submerged bending and axial vibration characteristics which duplicate those of a xe2x80x9clongxe2x80x9d prototype.
Other objects, advantages and features of this invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.